The High-Definition Multimedia Interface (HDMI) has become the prevalent specification for transmitting digital video and audio data from high bandwidth data sources to digital data presentation devices. HDMI sources such as DVD players, Blu-ray disc players, personal computers, video game consoles, etc., output video and audio data generated from these sources according to the HDMI specification. Digital data presentation devices capable of receiving the HDMI data are called HDMI sinks, and examples include digital televisions that may be either high-definition or non-high definition, computer monitors, laptop computers, smartphones, tablets, video game consoles and other devices.
The HDMI specification describes three separate communications channels: the Transition Minimized Differential Signaling (TMDS) channel, the Display Data Channel (DDC), and the Consumer Electronics Control (CEC) channel. The TMDS communications channel carries all audio and video data as well as auxiliary data that describes the active audio and video streams. An HDMI source device uses the DDC communications channel to determine the capabilities and characteristics of the HDMI sink device by reading the Extended Display Identification Data (EDID) data structure from the sink device. The EDID, which indicates the HDMI sink's capabilities, can include, by way of example, manufacturer name, product type, timings supported by the display, display size, luminance data and pixel mapping data. The HDMI source device reads the EDID and delivers only the audio and video formats that the HDMI sink device will support. The DDC also includes High-bandwidth Digital Content Protection (HDCP) data, which is a proprietary data stream that provides copy protection to the data supplied by an HDMI source to an authenticated HDMI sink. The CEC communications channel is optional, and provides support for higher-level user functions such as automatic setup tasks or the like.
FIG. 1 represents a system composed of a source device 1 connected to a sink device 2 through an HDMI cable 3. HDMI cable 3 contains TMDS lines for the transfer of data and control lines, serial clock (SCL) and serial data (SDA) lines and an optional CEC line for communication relating to control of the interface. The control lines SCL and SDA will henceforth be collectively referred to as the DDC (Display Data Channel) lines. The DDC connection is implemented in the HDMI specification using the “inter-integrated circuit” or I2C bus specification. An HPD (Hot Plug Detect) line serves to allow the source device 1 to detect the connection of the sink device 2. A DC line +5V is powered by source device 1 and is used by sink device 2 in order to establish reference voltage levels and to provide power to the HDMI interface circuits in the sink device 2.